A testable hypothesis is one that can be supported or refuted through experimentation or observation.
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Dropping a ball from a height of 10m will cause it to bounce back up 5m. - This is testable; you can measure the height to which the ball bounces.
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Plants produce glucose as the end product of photosynthesis. - This is also testable; you can measure glucose production in plants under controlled conditions.
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If certain foods are healthy for you, then they should also taste good. - This is subjective and not easily testable in a scientific manner, as "healthy" and "taste good" can vary based on individual perceptions.
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If acid is added to a solution, then the pH of the solution will go down. - This is testable; you can measure the pH before and after adding acid.
Based on these assessments, the following hypotheses are clearly testable:
- Dropping a ball from a height of 10m will cause it to bounce back up 5m.
- Plants produce glucose as the end product of photosynthesis.
- If acid is added to a solution, then the pH of the solution will go down.
So, the most definitive testable hypotheses in your list would be the first, second, and fourth options.